The present invention relates to an electric switch, and more particularly, to a microswitch with shifting gear to shift the switch on and off by a striking arm, which can be widely used in small current control system, such as limit system, alarm system, indicate system, etc. It is specially suited to set indication, alarm and urgent tripping in pointer type instruments, like pressure gauge, thermometer and so on.
Microswitches are widely used in many fields, in recent times, microswitch are in general typically operated by pressing an actuator (or a button). FIG. 1 shows a typical electric contact unit used in a thermometer. As shown in FIG. 1, the indicate pointer is fixed with the cam together. When the temperature increases, the pointer turns counterclockwise with the cam synchronously. When the temperature reaches a first set point, the cam causes the microswitch K1 to be depressed and activated because of the change of the cam""s diameter. When the temperature goes up continuously, the cam will cause microswitches K2 and K3 to be depressed and activated one by one; and the cam itself is bearing the reactive force of the restoring torque from the activated microswitches. The reactive force increases when the number of the activated microswitches increases, and the damping force to the pointer becomes heavier, which will influence the accuracy of temperature indication. When the activated microswitches reach a certain quantity (usually be 2), the indication error will be more than the acceptable range in many cases. In another respect, more force moment is required at the point xe2x80x9cOxe2x80x9d because the reaction force from the activated microswitches will affect the pointer""s rotational movement. Thus the miniaturization of the thermometer bulb is restricted.
To overcome the increasing reactive forces exerted from several microswitches, one method is to use a microswitch whose indicating pan is separated from the setting pan, and the indicating pointer is coupled to the setting pointer which activates the microswitch. Because the accuracy is influenced by both the driving device""s transmission error and the setting error caused by the separation of the setting pan and indicating pan, this method requires high techniques and the quality control is difficult.
Therefore, the purpose of the invention is to provide a microswitch which includes a shifting unit that can overcome the aggregation of reactive forces from the activated switches and is easy to manufacture, the microswitch is especially suited for use in pointer type instruments.
To achieve the above object, the present invention provides a microswitch comprising a switch unit, a pressure bar, an actuator, and a shifting gear.
The switch unit, pressure bar, and actuator are the same as or similar to a typical switch.
The shifting gear is fixed near one side of the pressure bar. This shifting gear is made of a cam and two forks fixed on the cam. The center shaft of the cam is fixed relative to the switch unit by a method generally known in the art. On the side of the cam facing the pressure bar, there are two contact points or two contact surfaces which are un-equidistant from the center of the cam. When a striking arm hits on the fork, the cam will rotate about the shaft and change its contact point or contact surface that is facing the pressure bar, thereby changing the functional mode of the switch.
The striking arm and the pointer are fixed on a same shaft and the striking arm swill shift the switch on or off when the pointer rotates pass the set point.
According to the above-mentioned method, the shaft of the pointer sustains a reactive force only when the striking arm hits on the fork to shift the microswitch on or off. The problem of aggregated pressure is overcome. Therefore, the pointer has a high indication accuracy and requires less force moment from the thermometer bulb. The structure of present invention is very simple, has no setting error, and is very practical.